The present invention relates to a receiver in an automobile subscriber station, a cordless telephone, or an FM tuner, and more particularly to an intermediate frequency amplifier stage used in such a receiver and equipped with a signal strength detection circuit.
An intermediate frequency amplifier stage typically comprises of a plurality of amplifiers in order to amplify the received very weak signal, and is further equipped with a circuit for detecting the strength of the received signal. The output of the signal strength detection circuit is used to judge the reception conditions or to drive a signal level meter. The signal strength detection circuit includes a plurality of rectifiers, each coupled to an associated amplifier in the intermediate frequency amplifier stage. The outputs of the respective rectifiers are added to derive the output of the signal strength detection circuit.
Each signal rectifier according to prior art has been a half wave rectifier circuit utilizing a capacitor and a base-emitter junction diode of a transistor, as disclosed in "Microelectronics and Reliability" Vol. 16, pp. 345 to 366, Pergamon Press, 1977. However, such half wave rectifier circuits are inferior in their rectification efficiency. For this reason, the added outputs of the rectifiers, that is, the signal strength detection output, has considerably poor linearity. If the number of the rectifiers is increased the linearity improves, but this involves an attendant increase in the number of intermediate frequency amplifiers. Moreover, each capacitor occupies a relatively large chip area and thus is not suitable for being formed as an integrated circuit. When the intermediate frequency becomes low, the capacitors are required to have a considerable capacitance, so that it becomes impossible to fabricate the capacitors in an integrated circuit.